A fuel cell is composed of a polymer electrolyte membrane which selectively transports protons, and a pair of catalyst electrodes (a fuel electrode and an air electrode) which sandwich the polymer electrolyte membrane. The fuel cell which has the above structure can continuously take out electric energy by supplying a fuel gas (hydrogen is contained) to the fuel electrode, and supplying an oxidizing gas (oxygen is contained) to the air electrode.
The polymer electrolyte membrane is composed of an electrolyte which contains a polymer ion-exchange membrane or the like, such as a sulfonic acid group-containing fluorine resin ion-exchange membrane or hydrocarbon resin ion-exchange membrane. In order for the polymer electrolyte membrane to have an ion transport function, it needs to contain a given quantity of water.
The catalyst electrode is composed of a catalyst layer that promotes a redox reaction therein and of a gas diffusion layer having air permeability and electric conductivity. The catalyst layer is positioned on the polymer electrolyte membrane side. The gas diffusion layer is composed of a carbon coat layer for improving adhesion to the catalyst layer and of a gas diffusion base layer through which a gas supplied from an external source is allowed to diffuse to the catalyst layer. The catalyst layer for the fuel electrode contains, for example, platinum or platinum-ruthenium alloy, and the catalyst layer for the air electrode contains, for example, platinum or platinum-cobalt alloy. An assembly of such a polymer electrolyte membrane and a pair of catalyst electrodes (each composed of a catalyst layer, a carbon coat layer, and a gas diffusion base layer) is called a membrane electrode assembly (hereinafter “MEA”).
MEAs can be electrically connected in series by stacking them on top of each other. At this time, in order to keep a fuel gas and an oxidizing gas from being mixed and to electrically connect each MEA in series, a conductive separator is disposed between each of the MEAs.
Separators include a fuel electrode separator which contacts a fuel electrode, and an air electrode separator which contacts an air electrode. Usually, fuel gas channels for supplying a fuel gas to the MEA are formed in the fuel electrode separator, and oxidizing gas channels for supplying an oxidizing gas to the MEA are formed in the air electrode separator.
In a conventional fuel cell, linear gas channels run in parallel to one another (see, e.g., Patent Literature 1). FIG. 1 is an exploded perspective view of the fuel cell disclosed by Patent Literature 1. The fuel cell illustrated in FIG. 1 has membrane electrode assembly 1, air electrode separator 2, and fuel electrode separator 3. Air electrode separator 2 has linear oxidizing gas channels 8 that run in parallel to one another.
In a fuel cell configured as described above, the gap between oxidizing gas channels, i.e., rib width, is generally small. Since ribs do not contribute to supply of an oxidizing gas unlike oxidizing gas channels, reducing rib width makes it possible to supply more oxidizing gas to the MEA. Therefore, in the conventional fuel cell, in order to increase the amount of oxidizing gas to be supplied, rib width is tended to be made small.
Moreover, as described above, the polymer electrolyte membrane needs to contain a given quantity of water for exerting permeability to ions. Therefore, in the conventional fuel cell, in order to ensure sufficient water content, fuel gas and oxidizing gas are humidified in advance. However, a humidifier for humidifying the oxidizing gas does not contribute to power generation directly; rather, it requires a space for installation. Therefore, development of a fuel cell cogeneration system that does not need any humidifier can achieve reductions in size as well as overall system cost reduction. Development of such a fuel cell cogeneration system can also avoid energy loss from the humidifier, thus increasing overall system efficiency. For these reasons, development of a fuel cell has been desired in which power generation efficiency does not decrease even when a less humidified or non-humidified oxidizing gas is employed.
As a conventional technology aiming to prevent the flooding phenomenon, which occurs when the water generated in the reaction gas channel during power generation remains, two or more discrete reaction gas channels are provided in the separator (see, e.g., Patent Literatures 2, 3 and 4). In the fuel cells disclosed by Patent Literatures 2, 3 and 4, each discrete reaction gas channel is formed as a serpentine channel.
As a conventional technology aiming to ensure the hardness of ribs that define reaction gas channels, two or more reaction gas channels are provided in parallel to one another (see, e.g., Patent Literature 5). The reaction gas channels of the fuel cell disclosed by Patent Literature 5 are made serpentine. The separator for the fuel cell disclosed by Patent Literature 5 is a carbon separator.